The present invention relates to the field of holographic data storage, and in particular to a system and method for storing data as micro-localized alterations in a holographic storage disk provided with a format hologram.
In conventional holographic storage, data is stored as holograms resulting from the interference of a signal and reference beam. During storage, both the reference and signal beams are incident on the storage medium. During retrieval, only the reference beam is incident on the medium. The reference beam interacts with a stored hologram, generating a reconstructed signal beam proportional to the original signal beam used to store the hologram.
For information on conventional volume holographic storage see for example U.S. Pat. Nos. 4,920,220, 5,450,218, and 5,440,669. In conventional volume holographic storage, each bit is stored as a hologram extending over the entire volume of the storage medium. Multiple bits are encoded and decoded together in pages, or two-dimensional arrays of bits. Multiple pages can be stored within the volume by angular, wavelength, phase-code, or related multiplexing techniques. Each page can be independently retrieved using its corresponding reference beam. The parallel nature of the storage approach allows high transfer rates and short access times, since as many as 106 bits within one page can be stored and retrieved simultaneously.
Conventional page-based volume holographic storage generally requires complex, specialized components such as amplitude and/or phase spatial light modulators. Moreover, ensuring that the reference and signal beams are mutually coherent over the entire volume of the storage medium generally requires a light source with a relatively high coherence length, as well as a relatively stable mechanical system. Mechanical stability and coherence-length requirements have hindered the development of inexpensive, stable, and rugged holographic storage devices capable of convenient operation in a typical user environment.
In U.S. Pat. No. 4,458,345, Bjorklund et al. describe a bit-wise volume holographic storage method using signal and reference beams incident on a rotating disk in a transmission geometry. The signal and reference beams are incident from the same side of the disk. The angle between the reference and signal beams can be altered to store holograms at various depths within the medium. A separate photodetector is used to retrieve data stored at each depth. The interaction of light with the medium is localized through two-photon recording.
In U.S. Pat. No. 5,659,536, Maillot et al. describe a system in which multiple holograms are stored at each location in a disk through wavelength multiplexing. Each hologram spans the depth of the medium. In U.S. Pat. No. 5,289,407, Strickler et al. describe a multi-layered, non-holographic, index-perturbation optical storage system. Bits are stored as localized perturbations in the index of refraction of a photopolymer, caused by the high intensity at the focus of a single laser beam.
Briefly, and in general terms, the present invention provides a multi-depth, bit-wise optical data storage and/or retrieval system using a holographic disk and method having improved storage density, and in which the optical components used for storage and retrieval can be relatively simple, inexpensive, and robust.
With the present invention, a format hologram is first stored in a holographic medium of a holographic storage device, and data are then stored as selective, microlocalized alterations of the format hologram. The alterations are stored at a plurality of depths within the medium, thereby allowing the storage of multiple data layers. The device uses a holographic medium which is flexible or hard and comes in the form of a disk or card.
Storing the format hologram, which requires maintaining mutual coherence between two light beams, can be performed in controlled conditions in a factory environment. Data storage and retrieval can then be performed in a user environment using a relatively simple and robust device. Storage or retrieval can be performed using a single light beam incident on the medium, and using a single optical head.
In a presently preferred embodiment, the format hologram is a reflection hologram stored throughout the volume of the medium, and having substantially planar fringes oriented parallel to a surface, e.g., the top surface, of a disk. The reflection hologram is capable of reflecting light traveling along the depth of the medium. Partial or complete deletion of the reflection hologram generates storage locations of lower reflectivity than the surrounding intact regions of the hologram. In one embodiment the disk can be made of flexible materials, i.e., the disk is a flexible or floppy disk
Other features and advantages of the invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, which illustrates by way of example the invention.